The objectives of this project are use vermilion (v), a gene required for eye pigment synthesis in Drosophila, a model system (1) to investigate the phenomenon of suppression of transposable element insertion mutations and (2) to investigate the molecular mechanisms which regulate the expression of the gene during development. The vermilion gene has been cloned and the structure of the gene has been determined in some detail. Mutations that disrupt v gene expression are clustered within approximately 2 kilobases of DNA. A 1.4 kb transcript, homologous to this same region, is present in v+ RNA and altered in either size or level of accumulation by various v mutations. The spontaneous v mutations that are suppressed by the suppressor of sable [su(s)] are apparently identical insertions of 412, a retrovirus-like transposable element. Transcription mapping and DNA sequencing experiments have shown that the v transcript consists of 5 or more exons. The positions of several mutations relative to introns and exons have been determined by sequencing. The suppressible 412 insertion mutations appear to be located within an intron near the 5' end of the gene. A revertant of one of the mutations is a secondary insertion of 2.6 kb of DNA into one end of the 412 element. The insertion of 412 has been shown to prevent acccumulation of the v transcript, and su(s) partially restores the level of v transcription. Comparison of the developmental profiles of vermilion transcription and enzyme activity indicates that the expression of the gene product is regulated both at the level of transcription and post-transcriptionally.